Correct version of Python in example config (circleci#5122)

Corrected the version of python specified in the compiled config snippet to match that of the parameter being passed in.

Author: ogii

jekyll/_cci2/reusing-config.md

@@ -895,7 +895,7 @@ jobs:
   build:
     steps: []
     docker:
-      - image: cimg/python:3.8
+      - image: cimg/python:2.7
         auth:
           username: mydockerhub-user
           password: $DOCKERHUB_PASSWORD  # context / project UI env-var reference